According to the EOS M manual, "This camera is a Type-A camera that can use all the features of EX-series Speedlites." I have only used the 90EX so far, but have plans to test the 550EX, 430EX II, and ST-E2.
A combination of optical viewfinder and rear touch screen exposure controls wouldn't work very well. Lack of an optical viewfinder doesn't bother me particularly. However, holding the camera away from your body with a long lens isn't ideal, and also the display can be difficult to see in bright sunlight.
For me, the decision was mainly about portability. In its smallest configuration, with just the EF-M 22mm lens, I can still carry the EOS M in a pouch on my belt. Other than that, I love the versatility of the system, being able to attach all my lenses.
AS PWP says, Canon needs to release a 20/22mm EF pancake. The EF 40mm pancake is a bit long as a general walkaround lens on an APS-C camera. Such a lens would be a good companion for the SL1.
I am not a video person, but video on the EOS M looks good and may encourage me to do more video.
The EOS M AF problems, even with firmware 1.0.6, have been wildly exaggerated. V2.0.2 looks to be a good improvement.
My choice was the EOS M. Only you can decide what will be best for your particular requirements.http://phil.uk.net/photography/canon_EOS_M.html